The present invention relates to a local area network with optical transmission. It is known that, in an industrial installation, a local area network provides links between sensors and/or actuators and automation systems such as controllers, programmable or otherwise, or regulators, etc. Such sensors, actuators, and systems constitute subscribers to the network. The data transmitted by a subscriber is typically transmitted to all of the other subscribers via optical fibers and via active optical star couplers performing optical coupling and constituting nodes of the network.
Frames carrying the data are transmitted at a data rate that is typically equal to 2.5 Mbit/s. Consideration is being given to raising the data rate to 5 or 10 Mbit/s.
A family of known local area networks is described in French Standard UTE C 46-607:
"FIP bus for performing data interchange between transmitters, actuators, and controllers--Base-band physical layer on optical fiber".
A local area network of this known family includes the following elements which, with respect to their functions indicated below, are common to a local area network of the present invention:
Terminal sets capable of transmitting and receiving optical signals. Such sets are in particular subscriber sets, each of which is connected, e.g. electrically, and assigned to a subscriber. PA1 Nodes constituted by active star couplers. PA1 Primary fibers optically connecting each of the nodes to sets associated with the node. PA1 Secondary fibers optically connecting each of the nodes to other ones of the nodes which are thus connected directly to the node. By means of the secondary fibers, each of the nodes is connected at least indirectly to all of the others. Via the fibers to which it is connected, each node is capable of receiving an incoming optical signal carrying data, and of responding by transmitting corresponding outgoing optical signals carrying the data to all of the associated sets and to all of the nodes connected directly to the node. For that purpose, the node includes optoelectrical transducers and electrical amplifiers. More precisely, for each association of an incoming optical signal and of a corresponding outgoing optical signal, the node includes: PA1 a single primary fiber connects each set to the associated node; PA1 the receive secondary transducers and the transmit secondary transducers are tuned to one of the two wavelengths constituting a common wavelength; PA1 a single secondary fiber interconnects two nodes when the two nodes are connected together directly; and PA1 means are provided so that, when a first node transmits an optical signal over the secondary fibers, an optical signal transmitted in response by a second node cannot interfere with the operation of the first node.
a receive transducer for transforming the incoming optical signal into an electrical reception signal; PA2 an amplifier for amplifying the electrical signal; and PA2 a transmit transducer for transforming the amplified electrical signal into an outgoing optical signal.
Each receive transducer or transmit transducer is referred to below as either a "primary" transducer if the optical signal is transmitted via a primary fiber, or a "secondary" transducer if the signal is transmitted via a secondary fiber.
Within the above known family, a local area network is characterized in particular by a wavelength which constitutes the wavelength of the network and to which all of the transducers of the network are tuned. Each node is connected to each of the sets that are associated with it via two optical fibers connected to respective ones of two primary transducers of the node, one of which transducers is a transmit transducer, the other transducer being a receive transmitter, and both transducers being assigned exclusively to the set.
For that purpose, the node includes a primary transmit transducer and a primary receive transducer for each of the sets that are associated with the node.
Each direct connection between two nodes of the network is provided by two secondary fibers which are assigned exclusively to interconnecting the two nodes. A first fiber guides the incoming signals of a first node and the outgoing signals of the second node. The second fiber guides the outgoing signals of the first node and the incoming signals of the second node. The outgoing signals of the first node (or of the second node) are transmitted via a secondary transmit transducer of the first (or of the second) node, and are received via a secondary receive transducer of the second (or of the first) node, the two transducers being assigned exclusively to interconnecting the two nodes. For that purpose, each node includes a secondary transmit transducer, and a secondary receive transducer for each other node connected directly to the node.
Such a known network suffers from the drawback that, even if the elements used to make up the network are as well chosen as possible, the number of subscribers that can be interconnected appears too limited to satisfy the needs of industry. Moreover, its cost is high.